Modulation and coding adjustment based on slant range characteristics for satellite downlinks

ABSTRACT

A satellite communications system adjusts a modulation and coding format for a downlink based on the slant range distance from the satellite to the receiving ground terminal. Modulation and coding formats are each associated with different sets of slant range distances. An estimated slant range from a satellite to a receiving ground terminal for a time period is determined, and the modulation and coding format associated with that slant range distance is identified. Data may then be transmitted by the satellite to the receiving ground terminal using the identified modulation and coding format for the time period. Additional factors may be considered in identifying the applicable modulation and coding format to be used.

CROSS REFERENCES

This application claims priority from U.S. Provisional PatentApplication No. 60/883,934 filed Jan. 8, 2007, entitled “PREDETERMINEDMODULATION AND CODING ADJUSTMENT FOR REMOTE SENSING SATELLITEDOWNLINKS”, which is incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to wireless communications in general and,in particular, to the adjustment of modulation and coding for satellitedownlinks.

Remote sensors are often placed on low earth orbit (LEO) satellites togather information about earth, weather, and space. Many remote sensorsgather information by sensing different wavelengths of electromagneticwaves emitted from an object of interest. One common use is satelliteimaging to map, measure, or otherwise monitor the earth. Other remotesensing systems may measure sounds, or variations in magnetic orgravitational fields. Remote sensing satellites are typically in lowearth orbit, and often transmit the data gathered with the remotesensors to ground terminals at locations on earth.

These transmissions may be bandwidth limited, and often operate within a400 MHz bandwidth in the X-band. This can present a limitation on theamount of data that may be transmitted to the ground terminals. Othertypes of satellite transmissions (e.g., with other types of sensors) mayhave similar bandwidth limitations. It may, therefore, be desirable toidentify novel methods that may be used to improve the bandwidthefficiency of a system for data transmission in a downlink of acommunications satellite.

SUMMARY

Satellite systems, devices, and methods are described to adjust amodulation and coding format for a downlink based on the slant rangedistance from the satellite to the receiving ground terminal. In oneembodiment, each of a number of modulation and coding formats isassociated with a different set of slant range distances. The slantrange from a satellite to a receiving ground terminal is determined, andthe modulation and coding format associated with that slant rangedistance is identified. Data may then be transmitted by the satellite tothe receiving ground terminal using the identified modulation and codingformat. In some embodiments, additional factors may be considered inidentifying the applicable modulation and coding format to be used.

In one set of embodiments, a satellite communications system includes aground terminal in communication with a low earth orbit (LEO) satellite.The ground terminal is configured to associate each of a number ofmodulation and coding formats with a different set of slant rangedistances. The ground terminal then identifies a slant range distancebetween the LEO satellite and the ground terminal. The ground terminalselects the set of slant range distances including the identified slantrange distance. The ground terminal identifies the modulation and codingformat associated with the selected set of slant range distances, andtransmits a signal identifying the selected modulation and coding formatto the LEO satellite.

The LEO satellite, in wireless communication with the ground terminal,receives the signal identifying the selected modulation and codingformat from the ground terminal. The LEO satellite transmits a signal(e.g., including remote sensing data) to the ground terminal utilizingthe selected modulation and coding format.

In another set of embodiments, the ground terminal is again configuredto associate each of a number of modulation and coding formats with adifferent set of slant range distances. A number of slant rangedistances between the LEO satellite and a receiving terminal may bedetermined for specific times of an orbital pass. The ground terminalthen identifies time periods associated with each set of slant rangedistances. The ground terminal associates each of the identified timeperiods with a selected one of the modulation and coding formats. Aschedule of the selected modulation and coding formats and associatedtime periods may be generated and transmitted to a LEO satellite. Usingthe schedule, the satellite may apply the selected modulation and codingformats at the associated time periods.

In the above sets of embodiments, a ground terminal may measure orotherwise estimate a signal quality metric (e.g., related to bit-errorrate, signal-to-noise ratio, propagation loss, weather or interferenceconditions, etc.) for a signal to be received from the LEO satellite.The selected modulation and coding format may be changed dynamically(e.g., in real-time) based on the measured signal quality. In addition,the distances attributed to the different sets of slant range distancesmay be varied based on the measured signal quality. Similarly, the timesassociated with modulation and coding formats may be modified based onthe signal quality metrics.

The ground terminal may transmit a changed modulation and coding formatto the LEO satellite due to the modification to the selected set ofslant range distances. The LEO satellite may utilize the changedmodulation and coding format on a current orbit over the groundterminal. The characteristics of the receiver and antenna at the groundterminal may be used in the determination of the particular modulationand coding format to be used. It is worth noting that, in other sets ofembodiments, functions described as being performed by the receivingground terminal may be performed, in whole or in part, by one or moreother ground terminals or by the LEO satellite.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the following drawings. In theappended figures, similar components or features may have the samereference label. Further, various components of the same type may bedistinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

FIG. 1 is a block diagram illustrating a satellite communications systemwith adjustable coding and modulation implemented according to variousembodiments of the present invention;

FIGS. 2A and 2B illustrate examples of a coding and modulation tablethat may be used to implement adjustable coding and modulation accordingto various embodiments of the present invention;

FIG. 2C illustrates an example of a coding and modulation schedule thatmay be used to implement adjustable coding and modulation according tovarious embodiments of the present invention;

FIGS. 3A and 3B are simplified block diagrams illustrating a gateway andcomponents thereof configured according to various embodiments of thepresent invention;

FIG. 4 is a simplified block diagram illustrating a DVB-S2 framingformat that may be used to implement adjustable coding and modulationaccording to various embodiments of the present invention;

FIG. 5 is a simplified block diagram illustrating transmitter componentsof a satellite configured according to various embodiments of thepresent invention;

FIG. 6 is a flowchart illustrating a method of adjusting a modulationand coding format based on slant range distances, according to variousembodiments of the present invention;

FIG. 7 is a flowchart illustrating a method of adjusting a modulationand coding format based on slant range distances and additional factors,according to various embodiments of the present invention.

FIG. 8 is a flowchart illustrating a method of establishing a timingschedule for adjusting a modulation and coding format based on slantrange distances, according to various embodiments of the presentinvention;

FIG. 9 is a flowchart illustrating a method of establishing a timingschedule for adjusting a modulation and coding format based on slantrange distances and additional factors, according to various embodimentsof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A satellite communications system is programmed to adjust a modulationand coding format for a downlink from the satellite to the receivingground terminal based on the slant range distance. In one embodiment,each of a number of available modulation and coding formats isassociated with a different set of slant range distances. For example,shorter slant range distances may be associated with higher ordermodulation and higher rate coding formats. Thus, shorter slant rangedistances may be paired with formats having greater information density.The slant range from a satellite to a receiving ground terminal for aparticular period of time is determined, and the modulation and codingformat associated with that slant range distance is identified. Data maythen be transmitted by the satellite to the receiving ground terminalduring that period of time using the identified modulation and codingformat.

This description provides example embodiments only, and is not intendedto limit the scope, applicability, or configuration of the invention.Rather, the ensuing description of the embodiments will provide thoseskilled in the art with an enabling description for implementingembodiments of the invention. Various changes may be made in thefunction and arrangement of elements without departing from the spiritand scope of the invention.

Thus, various embodiments may omit, substitute, or add variousprocedures or components as appropriate. For instance, it should beappreciated that in alternative embodiments, the methods may beperformed in an order different than that described, and that varioussteps may be added, omitted, or combined. Also, features described withrespect to certain embodiments may be combined in various otherembodiments. Different aspects and elements of the embodiments may becombined in a similar manner.

It should also be appreciated that the following systems, devices,methods, and software may each be a component of a larger system,wherein other procedures may take precedence over or otherwise modifytheir application. Also, a number of steps may be required before,after, or concurrently with the following embodiments.

Novel systems, methods, devices, and software are described which mayadjust the modulation or coding format for a downlink in a remotesensing satellite system. FIG. 1 is a block diagram illustrating anexample of a remote sensing satellite communications system 100configured according to various embodiments of the invention. While aremote sensing low earth orbit (LEO) satellite communications system isused to illustrate various aspects of the invention, it is worth notingthat the principles set forth herein are applicable to a variety ofother satellite and communications systems, as well.

In one embodiment, the satellite 105 is a LEO satellite configured withone or more sensors to receive and measure electromagnetic waves fromearth. In other embodiments, other types of earth, weather, or spacesensors known in the art may be used, including radiometers,photometers, or various other imaging or scanning mechanisms. Also,other geodetic and acoustic sensors may be used, and the sensors may bepassive or active. The data produced by the sensors may then betransmitted by the satellite 105 to one or more ground terminals 115through ground terminal antennas 110.

The satellite 105 may transmit a signal 120 with a modulation and codingformat adjusted to the link conditions (e.g., slant range distance and,perhaps, other factors) between the satellite and the ground terminalantenna(s) 110. A ground terminal 115 may be configured to receivesensing signals 120 from the satellite 105. In one embodiment, theground terminal antenna 110 is a parabolic reflector. However, theantenna 110 may be implemented in a variety of alternativeconfigurations. The ground terminal 115 may then transmit the receivedsensing data over a network (e.g., the Internet or any other network),or may otherwise process, store, or forward it.

The modulation and coding format for a signal 120 may be adjusted toaccount for different slant range distances and other factors, such asweather and interference conditions. Other adjustments are possible, aswell. For example, as illustrated in FIG. 1, different ground terminals115 may be different distances away from the direct path of thesatellite 105. Thus, the pass of a satellite that orbits directly abovea ground terminal 115 may be characterized as making a “ninety degreepass,” while those making passes further away may be characterized bythe degree above the horizon as they pass (e.g., “seventy degree pass”or forty-five degree pass”). The elevation degree pass for an orbitingsatellite may be used to identify the modulation and coding formatassociated with a particular slant range distance (e.g., a seventydegree pass satellite may have different modulation and codingassociations for a particular slant range distance than a forty-fivedegree pass satellite). Ground terminals 115 may also have differentcapabilities (e.g., different equalizers, amplifiers, etc.). Thus, insome embodiments, the modulation and coding format may be adjusted toaccount for the ground terminal 115 and antenna 110 capabilities, aswell.

In addition to receiving wireless signals from the satellite 105, aground terminal 115 may generate and transmit wireless signals 125 tothe satellite 105 via the antenna 110. For example, the ground terminal115 may be configured to process information on the timing and slantdistances associated with a future orbital pass. The ground terminal 115may generate a schedule for modulation and coding format adjustments,and transmit the generated schedule to the satellite 105 before anorbital pass. The ground terminal 115 may be configured to dynamicallymodify the schedule according to current conditions (e.g., interferenceor other signal quality conditions). The ground terminal 115 mayfunction as a gateway, generating and transmitting schedule informationto a satellite 105 on behalf of other ground terminals 115.Alternatively, a ground terminal 115 may simply function as a receivingterminal, and not necessarily transmit scheduling information to thesatellite.

Any number of packet addressing and formatting schemes may be used todirect packets accordingly, and otherwise provide data security on theuplink, downlink, or thereafter through the network. Variouschannelization schemes may be used, such as Time Division MultipleAccess (TDMA), Frequency Division Multiple Access (FDMA), OrthogonalFrequency Division Multiple Access (OFDMA), Code Division MultipleAccess (CDMA), or any number of hybrid or other schemes known in theart. A variety of physical layer transmission modulation and codingtechniques may be used with certain embodiments of the invention,including those defined with the DVB-S2 and WiMAX standards.

The satellite 105 may be configured to receive wireless signals from theground terminals, or other sources. The signals may be received viaantennas connected with the satellite 105. The satellite 105 may receiveand process signals received from a ground terminal 115 (e.g.,telecommands from a telemetry ground station), and modify its directionand speed of travel, its sensors, or its transmitters in accordance withthe received signals. As noted above, a satellite may also receive aschedule generated by a ground terminal 115 indicating the appropriatemodulation and coding formats to be used at different times.

As noted, in one embodiment the satellite 105 is a LEO orbiting aroundthe earth, passing ground terminals 115 as it orbits. FIG. 1 illustratesone such orbit, passing from a point low on the horizon, labeled point A130-a, to point B 130-b above ground terminal 115-a, and then continuingon past the ground terminal to point C 130-c to begin to descend backover the horizon. Given this orbit pattern, as a satellite 105 comesinto range of a ground terminal 110, the slant range distance betweenthe satellite 105 and ground terminal 115 (i.e., line-of-sight distancebetween them) will initially decrease, before again increasing as thesatellite continues across the horizon. As the slant range decreases,the modulation and coding format needed to transmit data reliably withina given reliability margin may change to a higher order modulation andhigher rate coding format.

As is evident to those skilled in the art, wireless signals decrease inpower as they travel over distances due to path loss. This attenuationmeans that a higher order modulation and coding format may besufficiently reliable over shorter distances, while a lower ordermodulation and coding format may be needed to attain the samereliability over longer distances. The term “higher order modulation andcoding format” may be used hereinafter to indicate a relatively higherorder modulation and higher rate coding format. Similarly, the term“lower order modulation and coding format” may be used hereinafter toindicate a relatively lower order modulation and lower rate codingformat (e.g., a QPSK1/4 modulation and coding format is a lower ordermodulation and coding format than QPSK9/10 modulation and codingformat).

Using a higher order modulation and coding format over the shorterdistances may increase the spectral efficiency, allowing more bits to betransmitted per symbol. This is illustrated Table 1, which showscalculations for purposes of example only.

TABLE 1 Pilot Spectral Modulation Coding Tones Efficiency QPSK 1/4 On0.49 QPSK 1/3 On 0.66 QPSK 2/5 On 0.79 QPSK 1/2 Off 0.99 QPSK 3/5 Off1.19 QPSK 2/3 Off 1.32 QPSK 3/4 Off 1.49 QPSK 4/5 Off 1.59 QPSK 5/6 Off1.65 QPSK 8/9 Off 1.77 QPSK  9/10 Off 1.79 8PSK 3/5 On 1.78 8PSK 2/3 On1.98 8PSK 3/4 On 2.23 8PSK 5/6 On 2.48 8PSK 8/9 Off 2.65 8PSK  9/10 Off2.68 16APSK 2/3 On 2.64 16APSK 3/4 On 2.97 16APSK 4/5 On 3.17 16APSK 5/6On 3.30 16APSK 8/9 On 3.52 16APSK  9/10 On 3.57

Therefore, according to various embodiments of the invention, thetransmitter on a satellite 105 may be configured to adjust themodulation and coding format based at least in part on the slant rangedistance. In one embodiment, the ground terminal 115 may generate atiming schedule before an orbital pass, and the schedule may betransmitted to the satellite before or during an orbital pass. Someportion of the adjustment parameters may be preprogrammed into atransmitter before deployment of a satellite 105, and may be modifiedonce a satellite is deployed (e.g., with commands from ground terminal115, or otherwise). Thus, while a satellite 105 may simply receive anadjustment schedule, it may be configured with additional processingcapabilities. In one embodiment, a satellite 105 may be configured tokeep track of ground terminal 115 location and slant range distance, andadjust the modulation and coding format to maintain a given reliabilitymargin. A satellite 105 may determine that an adjustment to a modulationand coding format is called for because of a change in slant rangedistance. The modulation and coding format may be adjusted accordingly,and an adjustment to the data rate may be implemented concurrently. Onemanner in which this may be accomplished is with a modulation and codingformat table, where different slant range distances are associated withdifferent modulation and coding formats.

By way of example, the process may be initiated by identifying a pathloss factor. The path loss factor may represent a calculation to beapplied that estimates the attenuation of a signal from a satellite to aground terminal based on distance. A number of such calculations areknown in the art, and may be used. In addition to the path loss factor,additional factors may be identified which modify the path loss factorbased on characteristics of a given ground terminal 115. Such factorsmay, for example, include weather patterns, natural and man madeobjects, interference conditions, and other factors specific to aparticular ground terminal 115. In other embodiments, otherconsiderations may be used to modify signal quality calculation, such astime of day, time of year, type of data, reliability requirements, QoSguarantees, etc. Using the path loss factor, reliability minimums, andperhaps the other factors, an association between sets of slant rangedistances and various modulation and coding formats may be established.

Continuing with the above example, the satellite 105 sensors collectinformation, which is processed to create sensing data. A slant rangedistance for the particular ground terminal 115 (i.e., the line of sightdistance from the satellite 105 to the ground terminal 115) at a time orset of times is determined. The determined slant range distance isapplied to the association between sets of slant range distances andvarious modulation and coding formats. The highest order modulation andcoding format that may transmit the data with sufficient reliability isidentified. In this way, the spectral efficiency can be tailored to thelink conditions (e.g., the slant rage distance) of the link between thesatellite and the particular ground terminal, and the data rate can bemodified accordingly.

Referring to FIG. 2A, an example of a table 200 is illustrated in theform of a block diagram. This type of modulation and coding format table200 may, for example, be used by scheduler unit on a ground terminal 115and/or a microprocessor on a satellite 105 to determine the modulationand coding format to be used for sensing data 120 to be transmitted overa given slant range distance. The table 200 contains a column listing anumber of modulation and coding formats 210. Each modulation and codingformat entry 210 corresponds to a set of distances 205. Thus, using theslant range distance attributed to a destination ground terminal 115, anentry for a set of distances 205 encompassing the slant range over theparticular link may be identified, and the appropriate modulation andcoding format may be selected. For example, if a destination link has aslant range distance that falls within distance g-h, the modulation andcoding format QPSK 3/4 may be used. In one embodiment, each set ofdistances is identified by calculating the highest order modulation andcoding format that can be transmitted within a certain reliabilitymargin (e.g., with a 10 mm/hr rain consideration).

In other embodiments, other metrics and indicators may be used inaddition to or in place of slant range distance. For example, 1)elevation degree for a satellite pass, 2) historic, current, or forecastweather patterns or conditions, 3) natural and man made objects, 4)historic, current, or estimated interference conditions, 5) otherfactors specific to a particular ground terminal 115 (e.g., antenna sizeor configuration, receiver component characteristics), and 6) quality ofservice (QoS) or traffic characteristics, may be factored into themodulation and coding format table. More global considerations mayinclude time of day, time of year, type of data, etc. It is also worthnoting that a number of other data structures may also be used to relateslant range to modulation and coding formats. In still otherembodiments, other information density parameters, in addition tomodulation and coding format changes may be added to further adapt asignal to environmental or other conditions.

Referring next to FIG. 2B, an example of a table 250 is illustrated inthe form of a block diagram. This modulation and coding format table 250may be the table 200 of FIG. 2A and, thus, may be used by a groundterminal 115 or a satellite 105 to determine the modulation and codingformat for a given set of slant range distances. The table contains acolumn listing a number of modulation and coding formats 210-a. Eachmodulation and coding format entry 210-a corresponds to a set ofdistances 205-a (and an elevation degree 255) for a satellite 105 makinga forty-five degree pass over a ground terminal 115. Thus, using theslant range distance attributed to a forty-five degree ground terminal115, an entry for a set of distances 205-a encompassing the slant rangedistance for a particular link (and time) may be identified, and theappropriate modulation and coding format may be selected. For example,if a destination link has a slant range distance 1400 km, the modulationand coding format 8PSK 3/5 may be used. For a satellite 105 making aninety degree pass over a ground terminal 115, the applicable modulationand coding format may be different even over the same slant rangedistance.

Referring next to FIG. 2C, an example of a table 275 is illustrated inthe form of a block diagram. This modulation and coding format table 275may be generated from the table 250 of FIG. 2B and, thus, may begenerated by a ground terminal 115 or a satellite 105 to determine themodulation and coding format adjustments at certain times. The tablecontains a column listing a number of modulation and coding formats210-b. Each modulation and coding format entry 210 corresponds to a setof times 280 for a satellite 105 passing at forty-five degrees over aground terminal 115 to transmit using certain modulation and codingformats. Thus, times for slant range distances in a given orbital passmay be identified by a ground terminal 115 or a satellite 105. Using theslant range distances and times, a schedule of adjustment to anappropriate modulation and coding format may be determined. If the table275 is generated by the ground terminal 115, it may be transmitted tothe satellite 105 before (or during) the orbital pass.

In different systems, the modulation and coding format identificationand adjustment timing may be undertaken by a range of components by theground terminal 115, the satellite 105, or any combination thereof. FIG.3A is a block diagram 300 illustrating a ground terminal including areceiver unit 305, a scheduler 310, a transmitter unit 315, and a memoryunit 320. For purposes of this description assume the system 100 ofFIG. 1. These components (305, 310, 315, 320) may be implemented, inwhole or in part, in hardware. Thus, they may be made up of one, ormore, Application Specific Integrated Circuits (ASICs) adapted toperform a subset of the applicable functions in hardware. Alternatively,the functions may be performed by one or more other processing units (orcores), on one or more integrated circuits. In other embodiments, othertypes of integrated circuits may be used (e.g., Structured/PlatformASICs, Field Programmable Gate Arrays (FPGAs), and other Semi-CustomICs), which may be programmed in any manner known in the art. Each mayalso be implemented, in whole or in part, with instructions embodied ina computer-readable medium (e.g., the memory unit 320), formatted to beexecuted by one or more general or application-specific processors.

In one embodiment, the receiver unit 305 receives signals 302 with dataincluding information on slant range distances and orbital pass times,and stores the information in the memory unit 320. The memory unit 320may also store an association between each of a number of modulation andcoding formats and a different set of slant range distances (this may bestored in the form of a table such as the table 200 of FIG. 2A). Thescheduler unit 310 may calculate slant range distances associated withthe satellite 105 at certain times. The scheduler unit 310 may utilizethe calculated slant range distances to identify time periods when thecalculated slant range distances fall within selected ones of thedifferent sets of slant range distances. The scheduler unit 310 may thenassociate each of the identified time periods with an applicablemodulation and coding format. The transmitter unit 315 may transmitsignals 322 with data identifying the modulation and coding formats andassociated time periods (e.g., in the form of a transmission schedule).

In another embodiment, the ground terminal 115 may be configured toschedule adjustment to modulation and coding formats based on slantrange distances and other factors. Consider the block diagram in FIG. 3Bof a system 325 including a ground terminal 115 with an exampleembodiment of the receiver unit 305-a. In the illustrated embodiment,the receiver unit 305-a includes a signal quality measurement unit 335,a signal quality information unit 340, a timing/path trajectory unit345, and a traffic classification unit 350.

The receiver unit 305-a may receive information regarding slant rangedistances of a particular satellite 105, timing of the orbital pass,elevation information, and a number of other types of data. Thisinformation may be a signal 302-a received from a satellite 105 via anantenna 110 connected with the ground terminal 115. Alternatively, thereceived information may be received from a source (e.g., a server)within a network 330 (e.g., via a terrestrial or wireless connection tothe Internet or other public or private network).

The signal quality measurement unit 335 may be configured to measure thesignal quality (e.g., bit error rate, signal-to-noise ratio) of a signalreceived from a satellite 105. Various methods of measuring signalquality are known in the art, and any of these methods may be so used ina manner consistent with this disclosure. The signal quality measurementmay be applied to the satellite 105 transmitting the measured signal, orto another satellite. In a deteriorating signal quality environment,lower order modulation and coding may be called for. Thus, distancesassociated with a set of slant range distances attributed to a givenmodulation and coding format may be lowered as interference increases.

The signal quality information unit 340 may be configured to receive andprocess information related to signal quality. For example, informationon historic, current, or forecast weather patterns or conditions may bereceived or measured. Also, information on historic, current, orestimated interference conditions (perhaps based on the weatherinformation, or natural or man made objects) may be received andprocessed. Various methods of measuring and processing these factorsrelated to signal quality are known in the art, and any of these methodsmay be so used in a manner consistent with this disclosure. In adeteriorating signal quality environment (e.g., deteriorating weather orother interference conditions), lower order modulation and coding may becalled for. Thus, distances associated with a set of slant rangedistances attributed to a given modulation and coding format may belowered as interference increases.

The timing/path trajectory unit 345 may be configured to receive andprocess information related to the path of a satellite and the timing ofeach orbital pass. For example, information related to the timing andposition of satellite may be received. Satellites with different orbitalpasses (e.g., ninety degree vs. forty-five degree passes) may havedifferent tables (e.g., table 200 of FIG. 2A) for associating slantranges with modulation and coding formats. Satellites passing lower onthe horizon may have different associations than those passing directlyabove for signal reception. Also, the information on the timing of theorbital passes may be stored in the memory unit 320 (e.g., so that thescheduler unit 310 may generate a schedule of modulation and codingformat adjustments to be transmitted to a satellite 105).

A traffic classification unit 350 may receive QoS, other trafficclassification information, and bandwidth information. This and theother information received by a receiving unit 305-a may be stored in amemory unit 320. Also, configuration information for the antenna andreceiver of a given receiver terminal may be stored in memory unit 320.Returning briefly to FIG. 3A, the scheduler unit 310 may generate arelational table or other data structure associating sets of slant rangedistances with certain modulation and coding formats for a particularsatellite 105. Signal quality measurements, weather, interference,receiver and antenna characteristics may each be used to change orrefine the distances associated with each modulation and coding format.The scheduler unit 310 calculates slant range distances associated withthe satellite 105 using the information on orbital times from the memoryunit 320. The scheduler unit 310 utilizes the calculated slant rangedistances to identify time periods when the calculated slant rangedistances fall within selected ones of the different sets of slant rangedistances. The scheduler unit 310 may associate each of the identifiedtime periods with the applicable modulation and coding format. Thetransmitter unit 315 may transmit signals 322 with data identifying themodulation and coding formats and associated time periods (e.g., in theform of a transmission schedule).

To implement the variable modulation and coding scheme described above,a variety of adaptive or variable modulation schemes known in the artmay be used. Turning to FIG. 4, the framing format 400 for a frame of aDVB-S2 system is illustrated. The framing format may, for example, beused as the framing format for transmitting sensing data from thesatellite 105 to the ground terminal 115 of FIG. 1. While a DVB-S2system is used as an example, the principles specified herein areapplicable to a range of systems.

In this embodiment, a base-band frame 420 is made up of a base-bandheader 405, a data field 410, and padding 415. Data in the data fieldmay include sensing data from the one or more sensors on a satellite105. The data field may include addressing information (e.g., IPaddress, MAC address) indicating the nodes within a network to which thepacket will be directed. As will be discussed in more detail below, thesize of the data field and padding may vary depending on the modulationand coding format selected for the frame to be constructed.

A given modulation and coding format is associated with the base-bandframe 420 (e.g., by accessing the modulation and coding format table 200of FIG. 2A). Interleaving and FEC encoding (e.g., BCH and LDCP,according to the coding format designation) may then be performed on thebase-band frame 420 to produce an encoded base-band frame 425, and outercoding parity bits 430 and inner coding parity bits 435 are appended toproduce a FECFRAME 440. The DVB-S2 specification provides that theFECFRAME 440 will be of fixed size regardless of the modulation andcoding format used (i.e., normal frame is 64,800 bits, and a shortenedframe is 16,200 bits), and this structure may be used in thisembodiment. However, in other embodiments, the FECFRAME 440 size mayvary according to the modulation and coding format selected for theframe, to produce frames of uniform duration in time.

Continuing with the framing format, the FECFRAME 440 is bit mapped tothe applicable constellation (e.g., QPSK, 8PSK, 16APSK, 32APSK,according to modulation and coding format designation), to produce aXFECFRAME 445 made up of symbols representative of the frame contents. APLHEADER 450 is added to the XFECFRAME 445, together forming the PLFRAME465. The PLHEADER 450 is made up of a start of frame (SOF) slot 455 of26 symbols, and a modulation and coding format (PLSCODE) slot 460 of 64symbols specifying the modcode and size (i.e., whether normal orshortened FECFRAME). The PLHEADER 450 is encoded. The PLFRAME 465 isthen baseband-shaped and quadrature-modulated, as well as amplified andupconverted to be transmitted from the satellite 105.

Using this scheme, a particular ground terminal 115 need not know inadvance the particular modcode being used. Instead, in one embodiment,only the PLHEADER 450 needs to be decoded, and the modcode can beascertained thereby. In this way, the ground terminal 115 can receivevarying modcodes without the need for two-way communication. It is worthnoting that the DVB-S2 system may be modified in a variety of ways, andthis system is illustrative only.

In different systems, the encapsulation and modulation techniquesdescribed above may be undertaken by a range of components. However, forpurposes of this description, assume the system 100 of FIG. 1, utilizingthe single carrier ACM waveform transmitted by a satellite 105 to aground terminal 115. FIG. 5 is a block diagram 500 illustrating asatellite 105-a including a receiving unit 505 (e.g., including sensors,and a receiver to process wireless communications received from a groundterminal 115), a microprocessor 510, a memory unit 515, a base-bandencapsulation unit 525, an encoding unit 530, a mapping unit 535, a PLframing unit 540, and an RF unit 545. These components (505, 510, 515,525, 530, 535, 540, and 545) may be implemented, in whole or in part, inhardware. Thus, they may be made up of one, or more, ApplicationSpecific Integrated Circuits (ASICs) adapted to perform a subset of theapplicable functions in hardware. Alternatively, the functions may beperformed by one or more other processing units (or cores), on one ormore integrated circuits. In other embodiments, other types ofintegrated circuits may be used (e.g., Structured/Platform ASICs, FieldProgrammable Gate Arrays (FPGAs) and other Semi-Custom ICs), which maybe programmed in any manner known in the art. Each may also beimplemented, in whole or in part, with instructions embodied in acomputer-readable medium, formatted to be executed by one or moregeneral or application specific processors.

In one embodiment, the satellite receives sensing data via the sensorsof the receiving unit 505, and passes the sensing data to themicroprocessor 510. Sensing data from different sensors may be groupedtogether, perhaps in a single packet or set of packets. The sensing datais forwarded to the base-band encapsulation unit 525, perhaps afterbeing processed, formatted, and stored at the memory unit 515.

The microprocessor 510 also determines or otherwise identifies themodulation and coding format to be used for the set of sensing data. Todo so, the microprocessor 510 may access the modulation and coding datatable 520 in memory 515 to identify the modulation and coding formatapplicable to a given time or slant range distance. The microprocessor510 may calculate a slant range distance or time, or may receive suchcalculations via transmissions from a ground terminal 115. Themodulation and coding data table 520 may, for example, be the modulationand coding format table 200 of FIG. 2A or the schedule table 275 of FIG.2C. Either table may be wirelessly received from a ground terminal 115via the receiving unit 505 of the satellite. Functions described withreference to the ground terminal 115 may be performed, in whole or inpart, by the microprocessor 510.

Note that, in other embodiments, other factors may also be considered inaddition to slant range distance in determining a modulation and codingformat to be used. For example, 1) satellite elevation degree, 2)historic, current, or forecast weather patterns or conditions, 3)natural and man made objects, 4) historic, current, or estimatedinterference conditions, 5) other factors specific to a particularground terminal 115 (e.g., antenna size or configuration, receivercomponent characteristics), and 6) quality of service (QoS) or trafficcharacteristics, may be factored into the modulation and coding datatable 520 either onboard the satellite 105-a or at the ground terminal115.

Returning to the base-band encapsulation unit 525, it identifies themodulation and coding format to be used for the ground terminal to whichthe sensing data is directed (e.g., from the modulation and coding datatable 520 via the microprocessor 510). The base-band encapsulation unit525 encapsulates the sensing data to produce a base-band layer frame(e.g., the base-band frame 420 of FIG. 4, including a header 405, a datafield 410, and padding 415). An encoding unit 530 encodes the packet inaccordance with the applicable coding (e.g., using BCH and LDCP,according to selected modulation and coding format), and may appendparity bits to produce an encoded frame. This may, for example, be aFECFRAME 440. The encoded frame then proceeds to the mapping unit 535,which maps the contents of the frame to the constellation of theapplicable modulation format (as dictated by the identified modulationand coding format) to produce a frame made up of symbols representativeof the encoded frame contents. An encoded physical layer header(indicative of the modcode used), e.g. PLHEADER 450, is added by a PLframing unit 540 to produce a physical layer frame. The physical layerframe 465 is then baseband shaped and quadrature modulated, and alsoprocessed by one or more amplifiers and an upconverter by the RF unit545, to be transmitted to the applicable ground terminal in a downlinksignal 550, perhaps via steerable antenna.

It is worth noting that the satellite 105-a of FIG. 5 may be configuredwith an antenna to receive telecommands from one or more groundterminals 115. Through such telecommands, the parameters for thecollection and transmission of the sensing data may be modified. In someembodiments, however, the telecommands are not implemented in real time,instead taking a longer time for implementation and processing (e.g.,within the microprocessor).

FIG. 6 is a flowchart illustrating a method 600 of adjusting amodulation and coding format for a satellite based on slant rangedistance, according to various embodiments of the present invention. Themethod 600 may, for example, be performed in whole or in part by thesatellite 105 or ground terminal 115 of FIG. 1, 3, or 5.

At block 605, a number of modulation and coding formats are eachassociated with a different set of slant range distances. At block 610,a slant range distance between a low earth orbit satellite and areceiving terminal is identified. At block 615, the set of slant rangedistances that include the identified slant range distance isdetermined. At block 620, the modulation and coding format associatedwith the selected set of slant range distances is identified.

FIG. 7 is a flowchart illustrating a method 700 of adjusting amodulation and coding format for a satellite transmission to a receivingterminal based on slant range distance and other factors, according tovarious embodiments of the present invention. The method 700 may, forexample, be performed in whole or in part by the satellite 105 or groundterminal 115 of FIG. 1, 3, or 5.

At block 705, propagation loss factors for transmissions between a LEOsatellite and receiving terminal are estimated based on the location ofthe receiving terminal (e.g., relative to the satellite) and current orhistoric weather patterns (e.g., for the location). At block 710,antenna characteristics for the receiving terminal are identified. Atblock 715, a quality of service metric associated with the particulardata traffic to be transmitted from the LEO satellite to the receivingterminal is identified.

At block 720, a number of modulation and coding formats are eachassociated with a different set of slant range distances and elevationdegrees based on the propagation loss factors, receiving antennacharacteristics, and quality of service metrics. Thus, for a given setof propagation loss factors, receiving antenna characteristics, andquality of service metrics, each set of slant range distances for asatellite at a given elevation degree may be assigned a particularmodulation and coding format. This association of modulation and codingformats with slant range distances may be in the form of a table, suchas the table 200 of FIG. 2A, or may be made using other relational datastructures.

At block 725, a current signal quality is measured from the LEOsatellite to the receiving terminal. At block 730, distances associatedwith some sets of slant range distances associated with respectivemodulation and coding formats are modified based on the signal qualitymeasurement. For example, with deteriorating signal quality, a lowerorder modulation may be called for, and thus distances associated withparticular modulation and coding formats may be shortened.

At block 735, a slant range distance is identified between a LEOsatellite and a receiving terminal. At block 740, it is determined thata selected one of the modified sets of slant range distances include theidentified slant range distance. At block 745, the modulation and codingformat associated with the selected set of slant range distances isidentified. At block 750, the identified modulation and coding format istransmitted to the LEO satellite.

At block 755, a degradation in a current signal quality metric ismeasured, the degradation exceeding a threshold. At block 760, a changedmodification and coding format is identified to address the signalquality degradation. At block 765, the changed modulation and codingformat is transmitted to the LEO satellite for use in a current orbit.

FIG. 8 is a flowchart illustrating a method 800 of setting time periodsfor adjusting a modulation and coding format for a satellitetransmission to a receiving terminal based on slant range distances,according to various embodiments of the present invention. The method800 may, for example, be performed in whole or in part by the satellite105 or ground terminal 115 of FIG. 1, 3, or 5.

At block 805, a number of modulation and coding formats are eachassociated with a different set of slant range distances. At block 810,certain periods of time are each associated with selected sets of slantrange distances. For example, the slant range distance between asatellite and a ground terminal may be calculated for specific points intime. Then, for each set of slant range distances, the times when thecalculated slant range distance falls within a given set of slant rangedistances may be accumulated to identify the time periods when asatellite is within the different sets of slant range distances. Atblock 815, the identified time periods are associated with theapplicable modulation and coding format.

FIG. 9 is a flowchart illustrating a method 900 of setting up schedulesfor adjusting a modulation and coding format for a satellitetransmission to a receiving terminal based on slant range distances andother factors, according to various embodiments of the presentinvention. The method 900 may, for example, be performed in whole or inpart by the satellite 105 or ground terminal 115 of FIG. 1, 3, or 5.

At block 905, propagation loss factors are estimated for transmissionsbetween a LEO satellite and a receiving terminal based on location andcurrent interference estimates. At block 910, antenna and receivercharacteristics are identified for the receiving terminal. At block 915,traffic type and bandwidth demands associated with data traffic areidentified.

At block 920, each of a number of modulation and coding formats are thenassociated with a different set of slant range distances based on thepropagation loss factors, receiving terminal characteristics, andtraffic type. At block 925, slant range distances between the LEOsatellite and the receiving terminal are determined for specific timesof an orbital pass. At block 930, time periods associated withparticular sets of slant range distances are determined based on theslant range distance timing determinations. At block 935, each of theidentified time periods are associated with a selected modulation andcoding format (e.g., based on the set of slant range distancesassociated with each format at block 920).

At block 940, a schedule of the selected modulation and coding formatsand associated time periods is generated. At block 945, the schedule istransmitted from the receiving terminal to the LEO satellite. At block950, a propagation loss metric is measured exceeding a threshold (e.g.,measured at the receiving terminal due to interference from a harshthunderstorm). At block 955, a changed timing schedule is generated toaddress the increased propagation loss. At block 960, the changedschedule is transmitted to the LEO satellite (e.g., by the receivingterminal) for use in a current orbit.

It should be again noted that the methods, systems, and devicesdiscussed above are intended merely to be exemplary in nature. It mustbe stressed that various embodiments may omit, substitute, or addvarious procedures or components as appropriate. For instance, it shouldbe appreciated that in alternative embodiments, the methods may beperformed in an order different than that described, and that varioussteps may be added, omitted or combined. Also, features described withrespect to certain embodiments may be combined in various otherembodiments. Different aspects and elements of the embodiments may becombined in a similar manner. Also, it should be emphasized thattechnology evolves and, thus, many of the elements are exemplary innature and should not be interpreted to limit the scope of theinvention.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, it will be understood by oneof ordinary skill in the art that the embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, algorithms, structures, and techniques have been shownwithout unnecessary detail in order to avoid obscuring the embodiments.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flow chart, a flow diagram, a data flow diagram,a structure diagram, or a block diagram. Although a flowchart maydescribe the operations as a sequential process, many of the operationscan be performed in parallel or concurrently. In addition, the order ofthe operations may be re-arranged. A process is terminated when itsoperations are completed, but could have additional steps not includedin the figure.

Moreover, as disclosed herein, the terms “memory” and memory unit mayrepresent one or more devices for storing data, including read onlymemory (ROM), random access memory (RAM), magnetic RAM, core memory,magnetic disk storage media, optical storage media, flash memory devicesor other machine readable media for storing information. The term“machine readable medium” includes, but is not limited to, portable orfixed storage devices, optical storage devices, wireless channels, a simcard, other smart cards, and various other media capable of storing,containing or carrying instructions or data.

Furthermore, embodiments may be implemented by hardware, software,firmware, middleware, microcode, hardware description languages, or anycombination thereof. When implemented in software, firmware, middlewareor microcode, the program code or code segments to perform the necessarytasks may be stored in a machine-readable medium such as a storagemedium. Processors may perform the necessary tasks.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. For example, the above elements may merely be a component ofa larger system, wherein other rules may take precedence over orotherwise modify the application of the invention. Also, a number ofsteps may be required before the above elements are considered.Accordingly, the above description should not be taken as limiting thescope of the invention.

1. A satellite communications system configured to change a modulationand coding format applied to a downlink according to slant rangedistances, the system comprising: a ground terminal configured to:associate each of a plurality of modulation and coding formats with adifferent set of slants range distances; identify a slant range distancebetween a low earth orbit satellite and the ground terminal; determine aselected one of the different sets of slant range distances includingthe identified slant range distance; identify a selected one of theplurality of modulation and coding formats associated with the selectedset of slant range distances; and transmit data identifying the selectedmodulation and coding format; and the low earth orbit satellite, inwireless communication with the ground terminal, and configured to:receive the selected modulation and coding format from the groundterminal; and transmit data to the ground terminal utilizing theselected modulation and coding format.
 2. The system of claim 1, whereinthe ground terminal is further configured to: measure a signal qualityfor a signal received from the low earth orbit satellite; and modifydistances associated with at least a subset of the different sets ofslant range distances based on the measured signal quality.
 3. Thesystem of claim 2, wherein, the ground terminal is further configured totransmit a changed modulation and coding format to the low earth orbitsatellite because the identified slant range distance is associated withthe changed modulation and coding format due to the modification to theselected set of slant range distances; and the low earth orbit satelliteis further configured to utilize the changed modulation and codingformat on a current orbit over the ground terminal.
 4. The system ofclaim 1, wherein the ground terminal is further configured to: estimateinterference conditions between the low earth orbit satellite and thereceiving terminal for a time period; and modify distances associatedwith at least a subset of the different sets of slant range distancesfor the time period based on the estimated interference conditions. 5.The system of claim 1, wherein the ground terminal is further configuredto: establish the different sets of slant range distances based at leastin part on characteristics of the ground terminal antenna and receiverconfiguration.
 6. A method of identifying a modulation and coding formataccording to slant range distances, the method comprising: associatingeach of a plurality of modulation and coding formats with a differentset of slant range distances, wherein the association is based at leastin part on signal quality measurements performed by a receivingterminal; identifying a slant range distance between a low earth orbitsatellite and a receiving terminal; determining a selected one of thedifferent sets of slant range distances including the identified slantrange distance; identifying a selected one of the plurality ofmodulation and coding formats associated with the selected set of slantrange distances; and transmitting the selected modulation and codingformat from the receiving terminal to the low earth satellite.
 7. Themethod of claim 6, further comprising: transmitting data to thereceiving terminal utilizing the selected modulation and coding format.8. The method of claim 6, further comprising: measuring a signal qualityfor a signal from the low earth orbit satellite to the receivingterminal; and dynamically changing the selected modulation and codingformat based at least in part on the measured signal quality.
 9. Themethod of claim 6, further comprising: estimating interference fromcurrent weather conditions for a signal transmitted between the lowearth orbit satellite to the receiving terminal; and transmitting thechanged modulation and coding format to be used based on the estimatedinterference.
 10. The method of claim 9, wherein, the selectedmodulation and coding format comprises a first modulation mode and afirst code rate; and the changed modulation and coding format comprisesthe first modulation mode and a second code rate.
 11. The method ofclaim 6, further comprising: estimating, for a future time period,whether a propagation loss from the low earth orbit satellite to thereceiving terminal will exceed a threshold; and modifying distancesassociated with at least a subset of the different sets of slant rangedistances for the future time period based on the estimated propagationloss.
 12. The method of claim 6, further comprising: establishing thedifferent sets of slant range distances based at least in part oncharacteristics of the receiving terminal antenna.
 13. A method foradapting a modulation and coding format according to slant rangedistances, the method comprising: associating each of a plurality ofmodulation and coding formats with a different set of slant rangedistances; identifying time periods associated with each of at least asubset of the different sets of slant range distances; associating eachof the identified time periods with a selected one of the modulation andcoding formats; generating a schedule of the selected modulation andcoding formats and associated time periods; and transmitting theschedule from a receiving terminal to a low earth orbit satelliteconfigured to transmit the selected modulation and coding formats at theassociated time periods.
 14. The method of claim 13, further comprising:transmitting data to the receiving terminal utilizing the selectedmodulation and coding format.
 15. The method of claim 13, furthercomprising: determining a plurality of slant range distances between alow earth orbit satellite and a receiving terminal at specific times ofan orbital pass, wherein the time periods associated with the differentsets of slant range distances are identified based at least in part onthe determined slant range distances.
 16. The method of claim 13,further comprising: estimating a propagation loss from a low earth orbitsatellite to a receiving terminal; and modifying at least a subset ofthe identified time periods based at least in part on the propagationloss estimate.
 17. The method of claim 13, further comprising: measuringa signal quality for a signal transmitted between a low earth orbitsatellite and a receiving terminal, wherein the measured signal qualitycomprises a signal-to-noise ratio, a bit error rate, an alternativemeasure of propagation loss, or other signal quality metrics; andestablishing the different sets of slant range distances based at leastin part on the measured signal quality.
 18. A terminal for transmittingdata identifying a modulation and coding format, the ground terminalcomprising: a memory unit configured to store an association betweeneach of a plurality of modulation and coding formats with a differentset of slant range distances; a scheduler unit, communicatively coupledwith the memory unit, and configured to: calculate slant range distancesassociated with a low earth orbit satellite over time; utilize thecalculated slant range distances to identify time periods eachassociated with selected ones of the different sets of slant rangedistances; and associate each of the identified time periods with aselected one of the modulation and coding formats; and a transmitterunit, communicatively coupled with the scheduler unit, and configured totransmit information comprising an identification of the modulation andcoding formats and associated time periods.
 19. The terminal of claim18, wherein the scheduler unit is further configured to: generate aschedule of the selected modulation and coding formats and associatedtime periods, wherein the schedule comprises the transmittedinformation.
 20. The terminal of claim 18, further comprising: areceiver unit configured to receive a signal transmitted according tothe identified modulation and coding formats during the associated timeperiods.
 21. The terminal of claim 20, wherein, the receiver unit isfurther configured to estimate a propagation loss associated with thereceived signal; and the scheduler unit, communicatively coupled withthe receiver unit, is further configured to modify the modulation andcoding format associated with at least a subset of one or more of theassociated time periods.